In the past, electricity generating power plants were typically operated continuously with few starts and stops over the plant's operating life. More recently, the economic effect of deregulating of the power generation industry in the United States has been to require a larger portion of electrical generating power plants to be cycled off line during periods of low electrical demand and to be cycled back on line during periods of high demand. For example, a power plant may be operated for a peak power demand period of 8 to 16 hours each day, then shut down when the peak power demand period ends. As a result, power plants, such as gas fired combined cycle plants, are being designed for cyclic operation.
One important factor that influences a power plant's projected design life (which typically may be 30 years) is the chemistry associated with the working fluids of the power plant, such as impurities in the steam used to drive a steam turbine. Accordingly, chemical impurity requirements, comprising Normal and Action Levels, that set limits for chemical impurities have been developed to ensure that a power plant remains undamaged due to over exposure to such chemicals during the plant's design life. Despite the migration towards cyclic operation of power plants, the Action Levels defined for a power plant's design life are typically based on the science and experience of continuously operating power plants. However, it has proven difficult to meet predetermined Action Level guidelines for cyclically operating plants, especially during startup of the plant when chemical impurities may exceed specified Action Levels. Increasingly, the economic benefit of operating power plants in cyclic modes may override a concern to ensure the plant is only operated within Normal and allowed Action Levels to avoid damage. A degree of damage that may shorten the life of a plant may be acceptable if an economic benefit of generating electricity under conditions that may cause damage to the plant (such as exceeding specified action level for a certain period of time) offsets an economic loss corresponding to a shortened life of the plant and/or an increased maintenance requirement. For example, it may be economically feasible to corrode a steam turbine of a power plant with impure steam to generate comparatively high cost power during a period of high demand.